A zero voltage switching (ZVS) or zero voltage transition (ZVT) bridge converter for driving a rectifier circuit. A clamp winding or inductor is placed in parallel with a resonant choke of the bridge converter. The clamp winding clamps the voltage at the resonant choke. Clamping the resonant choke in turn limits the voltage at the secondary winding of an output transformer. The output transformer is connected to the rectifier circuit. By limiting the voltage at the output transformer, the voltage at the rectifier circuit is limited as well.
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1. In a soft-switching DC/DC converter including a positive voltage input terminal and a negative voltage input terminal interconnected by a semiconductor switching device, a transformer, and an output rectifier circuit coupled to a secondary winding of the transformer, an apparatus for clamping the voltage across the output rectifier circuit comprising:
first and second clamp diodes connected in series across the positive and negative voltage input terminals; a resonant inductor coupled in series with a primary winding of the transformer; and a voltage clamp circuit, the voltage clamp circuit including an inductor mutually coupled with the resonant inductor, the inductor having a first terminal connected to one of an end or an intermediate tap of the resonant inductor and having a second terminal connected to a common node of the first and second clamp diodes, the voltage clamp circuit limiting the voltage at the output rectifier circuit.
9. A soft-switching DC/DC converter comprising:
a positive input terminal and a negative voltage input terminal; a semiconductor switching device interconnecting the positive and negative voltage input terminals; a transformer having a primary winding and a secondary winding; an output rectifier circuit coupled to the secondary winding of the transformer; first and second clamp diodes connected in series across the positive and negative voltage input terminals; a resonant inductor coupled in series with the primary winding of the transformer; and a voltage clamp circuit, the voltage clamp circuit including an inductor mutually coupled with the resonant inductor, the inductor having a first terminal connected to one of an end or an intermediate tap of the resonant inductor and having a second terminal connected to a common node of the first and second clamp diodes the voltage clamp circuit limiting the voltage at the output rectifier circuit.
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The present invention relates generally to soft-switching DC/DC converters and, more particularly, to an apparatus for clamping the voltage across a resonant choke to clamp or limit the voltage across the output rectifier devices in a soft-switching DC/DC converter.
Phase shifted, DC/DC bridge converters having zero voltage transition (ZVT) or zero voltage switching (ZVS) are commonly utilized to minimize switching losses. While the typical bridge converter system can be quite satisfactory for many applications, one problem that such bridge converters present is that excess voltage overshoot and severe ringing can occur at the output rectifiers. The output rectifiers are typically embodied as rectifier diodes, but may also be synchronous rectifiers made up of MOSFETs. The excessive overshoot and ringing is caused by the interaction of the reverse recovery process of the rectifier with the leakage inductance of the transformer and can degrade and subsequently cause failure of the output rectifier devices. A snubber circuit does not typically suitably address the excessive overshoot and ringing. Typically clamping of the switches is required.
One proposed solution to this situation can be found with respect to U.S. Pat. No. 5,198,969 to Redl. Redl inserts a pair of clamp diodes in series between the upper and the lower voltage rails. The clamp diodes connect at a node which is between a choke inductor and an output transformer of the bridge converter. In effect, Redl clamps the voltage across the output rectifier devices to the voltage at the junction of the resonant inductor and the power transformer. This effectively clamps the voltage across the primary winding of the transformer and, consequently, the secondary winding of transformer as well. This topology, however, produces a substantial forward current and substantial reverse recovery current in the clamping diodes. This results in substantial power dissipation in the diodes.
An improvement upon the system of Redl can be found with respect to U.S. Ser. No. 09/798,186, filed Mar. 1, 2001, entitled Passive Voltage Clamp for Rectifier Diodes in a Soft-Switching DC/DC Converter, naming Guerrera as an inventor, which is assigned to the assignee of the present invention, the disclosure of which is incorporated by reference. Guerrera clamps both the primary and the secondary voltages at the power transformer by providing an additional winding, referred to as a clamp winding, at the power transformer. While the system of Guerrera proves effective, the manufacture of such a system introduces several challenges. In particular, designing a transformer which can be suitably built to incorporate the Guerrera design proves to be a challenge. Because Guerrera calls for an extra clamp winding in proximity to the primary winding of the transformer, configuring the transformer to include this additional clamp winding can reduce the efficiency of the transformer.
Thus, it is desirable to provide a system which limits excessive voltage overshoot and severe ringing at the output rectifier diodes while maintaining a relatively efficient, manufacturable ZVS/ZVT bridge converter.
This invention is directed to a soft-switching DC/DC converter. The converter includes a positive voltage input terminal and a negative voltage input terminal interconnected by a semiconductor switching device. A transformer of the converter includes a primary and a secondary winding, and an output rectifier circuit is coupled to the secondary winding of the transformer. The converter includes an apparatus for clamping the voltage across the output rectifier circuit. The apparatus includes first and second clamp diodes connected in series across the positive and negative voltage input terminals. A resonant inductor is coupled in series with a primary winding of the transformer, and a voltage clamp circuit is connected across the resonant inductor. The voltage clamp circuit limits the voltage at the output rectifier circuit.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
With reference to
Primary winding n1 is connected in series with a resonant choke Np of transformer T2 between the positive terminals of capacitors C1 and C2. The secondary windings n2a and n2b connect to output rectifier diodes D1 and D2. Secondary windings n2a and n2b share a common terminal which defines a negative reference voltage terminal for the voltage output Vo. The other terminals of secondary windings n2a and n2b connect to the anodes of respective diodes D2 and D1. The rectifier diodes D1 and D2 are connected to output inductor Lo. An output capacitor Co is connected across the voltage output Vo terminals, with one terminal of output capacitor Co connected to a terminal of inductor Lo.
Also with reference to
In operation, if the original number of turns of resonant choke Np is x turns, then the number of turns of voltage clamp Ncl is preferably selected to be equal to the number of turns of Np, or x turns. With the number of turns of resonant choke Np equal to the number of turns of the clamp winding Ncl, the voltage across resonant choke Np will be equal to the voltage across clamp winding Ncl. Assuming that the forward voltage drop of the diode D3 is Vd3 and the voltage drop of diode D4 is Vd4, then both the terminals of transformer T2 indicated by dots will be clamped during the positive excursion of the voltage to Vin+Vd3 and will be clamped during the negative excursion to the voltage-Vd4. If the diode drops are neglected, then the interconnection between the transformer T1 and the resonant choke Np will be clamped to Vin during the positive excursion of the voltage and to GND during the negative excursion of the voltage. As a result, the voltage across primary winding n1 of power transformer T1 will be clamped to an absolute value of Vin and consequently clamp the voltage across secondary winding n2 in accordance with the relative number of turns between primary winding n1 and secondary winding n2.
The present invention also offers several additional advantages. First, it is typically difficult to incorporate extra windings in the power transformer, especially for transformers utilizing sandwich winding techniques, were two halfprimary windings sandwich the secondary. Placing a clamp winding on the resonant choke typically alleviates this consideration. Second, if the transformer construction does not enable a transition to a bigger transformer, incorporating a clamp winding in the transformer can decrease the copper area of the windings, which will consequently decreased efficiency. Placing a clamp winding in the resonant choke alleviates this consideration. Thirds, when a clamp winding is placed in a power transformer, the leakage inductance between the primary winding and the secondary winding increases because of the increased separation between the primary and the secondary windings to accommodate the clamp winding. This requires a larger resonant choke for proper ZVS/ZVT switching. However, placing a clamp winding in the resonant choke will not increase leakage inductance of the power transformer. Fourth, for applications having more than one power transformer, only one clamp winding needs to be added to the circuit of
With reference to
With reference to
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
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